204 EARTH FEATURES AND THEIR MEANING 



though they may not occur for a year or more, come as sudden 

 downpours of great volume and violence. Rock masses, which 

 are highly heated beneath the desert sun, if suddenly dashed 

 with water, may be rent apart by the differential strains set up 

 near the surface. That rocks may be easily rent as a result of 

 sudden chilling is well known to our Northern farmers, who are 

 accustomed to rid themselves of objectionable bowlders by first 

 building a fire about them and then dashing water upon their 

 surface. Thus split into fragments, even the larger bowlders 

 may be handled and so removed from the farming land. The 

 natural process of rock rending by the occasional cloudburst may 

 be described as diffission. Blocks as much as twenty-five feet in 



diameter have been observed 

 in the desert of western 

 Texas, soon after being 

 broken into several frag- 

 ments at the time of a down- 



P ur of rain ( Fi g- 211). 



The natural sand blast. 

 Because of the saucer-like 

 shape, the vast expanse, and 



FIG. 211. Granite blocks in the Sierra de the absence of wind breaks, 

 los Dolores of Texas, rent into several the potency of wind as a 

 fragments by the dash of rain (after ^ ^ , -i 



Waither). geological agent is in desert 



areas not easily overesti- 

 mated. While most of its work is accomplished with the aid of 

 tools, it has been proven that even without this help, considerable 

 work is done through the friction of the wind alone, particularly 

 when moving as powerful eddies in cracks and crannies. This 

 wear of the wind, unaided by cutting tools, is known as deflation. 



The greater work of the wind is, however, accomplished with 

 the aid of larger or smaller rock particles, the sand and dust, 

 with which it is so generally charged above the deserts. Un- 

 protected by any mat of vegetation the materials of the desert 

 surface are easily lifted and are constantly migrating with the 

 wind. The finest dust is raised high into the air, and is carried 

 beyond the marginal barriers, but none of the sand or -coarser 

 materials ever passes beyond the borders. 



The efficiency of this sand as a cutting tool when carried by the 



